E. N. Economou, “Surface Plasmons in Thin Films,” Phys. Rev. 182:539 (1969); A. Taflove, “Computational Electromagnetics: The Finite-Difference Time-Domain Method,” Boston, Mass.: Artech House (1995); Veronis and Fan, “Bends and Splitters in Metal-Dielectric-Metal Subwavelength Plasmonic Waveguides,” Appl. Phys. Lett. 87:131102 (2005); Pile and Gramotnev, “Adiabatic and Nonadiabatic Nanofocusing of Plasmons by Tapered Gap Plasmon Waveguides,” Appl. Phys. Lett. 89:041111 (2006); and Heras, et al., “Direct Measurement of Frequency and Polarization Dependences of Cross-Phase Modulation in Fibers From High-Resolution Optical Spectra,” Opt. Lett. 31:14 (2006), are herein incorporated by reference in their entirety.
The ability to squeeze light to be ultra small is critical to high density optical interconnection, sensitive modulators, optical data storage, compact sensors, manipulation of nanostructures, sharper microscopy, and optical lithography in semiconductor industry. The extremely high light intensity resulting from the ultra small spot will greatly increase the nonlinear effect and can be used to make ultra small and ultra fast electric-optic or all-optic modulators. Recent progress in plasmonics offers new insight into this topic. (E. Ozbay, “Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions,” Science 311:189 (2006); Barnes et al., “Surface Plasmon Subwavelength Optics,” Nature (London) 424:824 (2003), which are hereby incorporated by reference in their entirety).
Currently, there are two approaches to squeeze light into a subwavelength scale using plasmon-based media. The first one is based on the small mode size that is supported by plasmon-based media in which light can be squeezed into a subwavelength aperture or propagated in a subwavelength waveguide. Directly coupling light into a deep subwavelength circular or square aperture has been tried and shown to have very low efficiency. As a result, more recent work has focused on transmitting light through deep subwavelength slits or coupling light into waveguides with deep subwavelength dimension only in one direction.
Extraordinary optical transmission was first observed through arrays of subwavelength holes. Each hole has a diameter (150 nm) slightly smaller than the diffraction limit of light (λ=326 nm) (Ebbesen et al., “Extraordinary Optical Transmission Through Sub-Wavelength Hole Arrays,” Nature (London) 391:667 (1998), which is hereby incorporated by reference in its entirety). The transmission through the aperture can be enhanced by fabricating periodic grooves surrounding the apertures (García-Vidal et al., “Multiple Paths to Enhance Optical Transmission through a Single Subwavelength Slit,” Phys. Rev. Lett. 90:213901 (2003), which is hereby incorporated by reference in its entirety).
Following the same principle, beaming light from a single subwavelength aperture was reported (Lezec et al., “Beaming Light from a Subwavelength Aperture,” Science 297:820 (2002), which is hereby incorporated by reference in its entirety). Two types of apertures were used in this work: a circular aperture with diameter 250 nm, which is slightly smaller than the diffraction limit for visible light and a slit aperture with deep subwavelength dimension in one direction, 40 nm, but in another dimension 4400 nm. If another deep subwavelength confinement by metal is applied, a cutoff frequency will be imposed and the transmission is extremely small. Resonant optical antennas considerably shorter than one-half the wavelength were shown to enhance field in the antenna feed gap and lead to white-light supercontinuum generation (Muhlschlegel et al, “Resonant Optical Antennas,” Science 308:1607 (2005), which is hereby incorporated by reference in its entirety). However, the low coupling efficiency and side lobes constitute significant drawbacks for practical applications.
Light propagation along a chain of gold particles with dimensions 100×100×40 nm3 deposited on an ITO substrate was observed in the visible light regime (λ=633 nm) (Krenn et al., “Squeezing the Optical Near-Field Zone by Plasmon Coupling of Metallic Nanoparticles,” Phys. Rev. Lett. 82:2590 (1999), which is hereby incorporated by reference in its entirety). Yin et al. (Yin et al., “Subwavelength Focusing and Guiding of Surface Plasmons,” Nano Lett. 5:1399 (2005), which is hereby incorporated by reference in its entirety), demonstrated light (λ=532 nm) guiding along a silver strip with a cross section of 250×50 nm2. There is also a report on very low-loss light propagation (˜100 μm) along triangular 0.6 μm wide and 1 μm deep gold grooves at a telecom wavelength (Bozhevolnyi et al., “Channel Plasmon-Polariton Guiding by Subwavelength Metal Grooves,” Phys. Rev. Lett. 95:046802 (2005), which is hereby incorporated by reference in its entirety). Numerical simulation of a nanowire taper (M. I. Stockman, “Nanofocusing of Optical Energy in Tapered Plasmonic Waveguides,” Phys. Rev. Lett. 93:137404 (2004), which is hereby incorporated by reference in its entirety) and experimental demonstration of a planar taper (Verhagen et al., “Nanofocusing in Laterally Tapered Plasmonic Waveguides,” Opt. Express 16: 45 (2008), which is hereby incorporated by reference in its entirety) were recently reported, where photons are converted into surface plasmon polaritons and propagate along the surface of a tapered nanowire or waveguide. However, how to efficiently couple light into such a chain, groove, and tapers, as well as how to decrease surface scattering are problems remaining unsolved.
The other approach to squeeze light subwavelength is based on negative refraction by plasmon-based media. Negative refraction can be employed to amplify and restore evanescent waves, which carry the fine information of the object, making super imaging resolution (Shin and Fan, “All-Angle Negative Refraction for Surface Plasmon Waves Using a Metal-Dielectric-Metal Structure,” Phys. Rev. Lett. 96:073907 (2006); Lu et al., “Three-Dimensional Subwavelength Imaging by a Photonic-Crystal Flat Lens Using Negative Refraction at Microwave Frequencies,” Phys. Rev. Lett. 95:153901 (2005), which are hereby incorporated by reference in their entirety). Sub-diffraction-limited optical imaging was obtained with a silver superlens by negative refraction with resolution of λ/6 (λ=365 nm). However, subwavelength images themselves require subwavelength objects (Fang et al., “Sub-Diffraction-Limited Optical Imaging with a Silver Superlens,” Science 308:534 (2005), which is hereby incorporated by reference in its entirety).
On the other hand, efficient light coupling from dielectric waveguides into plasmonic waveguides was numerically investigated in recent work. The key issue is to match the effective transmission cross section (determined by impedance and mode profile) of the plasmonic waveguides. It has been shown that the effective transmission cross section of a metal-dielectric-metal (MDM) waveguide is surprisingly much larger than the geometrical dimension of the dielectrics between metal slabs. This helps the transmission cross section match between a dielectric waveguide and MDM waveguide. The reason for this is still not completely clear. The light transmission enhancement on nanoscale antennas or by periodic textures may partially explain the high transmission (Muhlschlegel et al., “Resonant Optical Antennas,” Science 308:1607 (2005); Lezec et al., “Beaming Light from a Subwavelength Aperture,” Science 297:820 (2002); Gay et al., “The Optical Response of Nanostructured Surfaces and the Composite Diffracted Evanescent Wave Model,” Nature Phys. 2:262 (2006); Weeber et al., “Optical Near-Field Distributions of Surface Plasmon Waveguide Modes,” Phys. Rev. B 68:115401 (2003), which are hereby incorporated by reference in their entirety). Two-dimensional finite-difference time-domain (FDTD) simulations were performed (Veronis and Fan, “Theoretical Investigation of Compact Couplers Between Dielectric Slab Waveguides and Two-Dimensional Metal-Dielectric-Metal Plasmonic Waveguides,” Opt. Express 15:1211 (2007), which is hereby incorporated by reference in its entirety) for light direct coupling from a dielectric waveguide into an MDM waveguide with efficiency 68% (including propagation loss). The two-dimensional (2D) simulations promise to be valid for plasmonic waveguides with large dimensions in the third direction.
These research results are exciting and indeed constitute breakthroughs towards deep subwavelength photonics. However, they either provide deep subwavelength dimension only in one direction, or need the assistance of periodic textures, or require very complicated coupling configurations.